The present invention relates to a method of determining an inhomogeneous roadway in a driving situation of vehicles, which are disposed on a roadway having sidewise different coefficients of friction, during active ABS control and active yaw torque control (YTC) of a front wheel (HM-wheel) on the high-coefficient-of-friction side, and to a method of generating an additive additional moment at the steering wheel or at the wheels of a vehicle, in which case the additional moment is applied depending on a driving situation to a roadway with sidewise different coefficients of friction (μ-split).
During braking on inhomogeneous roadways (i.e. roads with different coefficients of friction on the left and the right vehicle side), asymmetrical brake forces occur due to the different coefficients of friction (right-left). The result of the asymmetric brake forces is a yaw torque around the vertical axis of the vehicle, which causes the vehicle to carry out a yaw movement towards the side of the road with the higher coefficient of friction (see FIG. 1).
Vehicles, which are not equipped with the electronic brake system ABS, become unstable in such driving situations since the cornering force of the tires gets lost when the wheels block. The yaw torque resulting from the asymmetrical brake forces causes the vehicle to turn quickly around its vertical axis towards the side with the high coefficient of friction (swerve).
In order to prevent the wheels of the motor vehicle from blocking in consequence of excessive brake pressure generated by the vehicle operator when the brake is applied, with the motor vehicle losing its stability or its steerability as a result, the hydraulic brake system of the vehicle is equipped with an anti-lock control device, i.e. it is configured as a hydraulic brake system with anti-lock control.
When an imminent locked condition of one or more of the vehicle wheels is detected, an anti-lock control device is used to automatically modulate the brake pressure independently of the brake pedal force generated by the operator at least in part of the hydraulic brake system, i.e. the brake pressure is decreased, maintained constant and re-increased long until the tendency to lock no longer prevails. Hence follows that the general objective of anti-lock devices for hydraulic motor vehicle brake systems is to safeguard the directional stability and the steerability of the vehicle as well as shortest possible stopping or braking distances, in particular on slippery roadways and with maximum operation of the service brake system (e.g. during panic stops).
On roadways with coefficients of friction on the right/left side (μ-split) of a remarkably differently high rate, however, the directional and driving stability will be reduced due to the very differently high rates of effective brake forces on the right and the left vehicle wheels. This major asymmetry and imbalance of the effective brake forces on the right and the left vehicle sides will generate a more or less great yaw torque that turns the vehicle about its vertical axis depending on these asymmetric forces. To counteract this condition and preserve the directional and driving stability, i.e. to keep the vehicle on course, the operator would have to manipulate the steering wheel in this situation with an extremely quick reaction for correcting purposes, but even skilled drivers will manage to do so in such emergency situations only rarely to a more or less satisfying degree.
Thus, there is a general conflict of goals in such situations for anti-lock controlled hydraulic brake systems for motor vehicles. One objective is to achieve maximum short braking or stopping distances, on the one hand, however, it is also important to maintain the directional and driving stability as well as the steerability of the vehicle in a braking maneuver, on the other hand.
Therefore, it has meanwhile become the generally binding philosophy of the manufacturers and users of anti-lock controlled hydraulic motor vehicle brake systems to give priority to the preservation of the directional and driving stability as well as the steerability of the vehicle over reaching shortest possible stopping distances.
The ABS control strategy is adapted in such driving situations in order to maintain the directional and driving stability of the vehicle. In this case, at least the two rear wheels undergo anti-lock control according to the so-called ‘Select-Low’ principle, i.e. depending on the vehicle wheel that is operated with the lowest coefficient of friction at that moment. This implies that in the service situation described above only the same comparatively low brake pressure is applied to the brake of the rear wheel rotating at the higher coefficient of friction μ that is applied to the brake of the other rear wheel that rotates on the lower coefficient of friction, although the first-mentioned wheel could be braked to a greater degree without locking on account of its higher coefficient of friction. Thus, equally great or equally low brake forces are applied to both rear wheels so that these do not contribute to yaw torque generation. Since the rear wheel rotating at the higher coefficient of friction is braked to a less intense degree than possible, this wheel possesses a correspondingly high potential to govern lateral forces what is of benefit to the directional and driving stability of the vehicle.
The price paid for the good directional and driving stability implies in each case longer stopping distances since the vehicle wheels rotating at higher coefficients of friction are braked at a reduced rate in this control principle than the adhesion coefficient prevailing in this case would per se allow.
If the two front-wheel brakes are anti-lock protected individually by devices of their own in an anti-lock hydraulic motor vehicle brake system with rear wheels being anti-lock controlled according to the ‘Select-Low’ principle, it is conventional practice to weaken the effect of yaw torque, which possibly develops due to differently great brake forces on the right and the left front wheels, by a so-called ‘Yaw Torque Limitation (YTL)’ that is superimposed on the individual anti-lock control of the two front wheels. The overriding ‘Yaw Torque Limitation’ ensures that the brake pressure at the front wheel (HM wheel) rotating at the higher coefficient of friction builds up more slowly than it would per se be possible in order to use the resulting delayed buildup of yaw torque to give the operator additional time to react, i.e. for countersteering. Of course, the overriding ‘Yaw Torque Limitation’ also contributes to an additional certain worsening of the attainable braking and stopping distance.
Document 39 25 828 A1 discloses anti-lock control with YTL which, for the determination of the pressure difference, measures the pressure on the right and left wheel introduced by the driver and determines the admissible pressure difference by way of a comparison between the nominal pressure and the actual pressure. Moreover, DE 41 14 734 A1 describes an anti-lock system with YTL, which manages without pressure sensors and, based on pressure reduction signals, continuously determines a value representative of the pressure difference of the two wheels of the one axle.
Furthermore, DE 44 41 624 A1 discloses a ‘Yaw Torque Limitation’ which starts a special control mode during braking maneuvers on roadways with μ-split patches. The differences in brake pressure reduction on the front wheels, the vehicle speed, the slip of the LM front wheel, the period of instability of the LM front wheel, and the HM wheel are assessed as prescribed criteria for the activation of the special mode conditions.
There is no special μ-split detection in an anti-lock control with ‘Select-Low’ and YTL, FIGS. 2a and 2b showing its principal pressure variations. The result is that frequently YTL is detected, even if the vehicle is braking on a homogeneous roadway. When one front wheel enters ABS control, pressure is stopped immediately on the other front wheel, the HM wheel. A so-called sympathy reduction pulse can be carried out after the pressure stop at the high coefficient-of friction wheel when the wheel behavior on the low coefficient-of friction side exhibits a certain dynamics. Further, the difference in pressure develops only slowly. A more precise detection of the inhomogeneous roadway is unnecessary in the YTL because the described behavior does not cause any essential loss in brake efficiency or loss in stability. The two front wheels have the same locking pressure level because the roadway is homogeneous. Therefore, the alleged high coefficient-of-friction wheel is close to the locking pressure level. This is why an undesirable YTL intervention is acceptable.
A motor vehicle with an anti-lock hydraulic brake system with YTL is known in the art (DE 40 38 079 A1). This system compensates the yaw torque developing in a μ-split driving situation during ABS control in that a compensation steering angle, which depends on the difference between the separately controlled brake pressures, is adjusted or superposed on the steering angle predetermined by the driver, respectively. The autonomous compensation steering angle improves the maneuverability in a braking maneuver on inhomogeneous roadways. The μ-split driving situation is determined based on the measured brake pressures or the brake pressures assessed by an assessing algorithm by way of the output data of the ABS valve actuation times.
This μ-split detection for vehicles with active steering, which is based on the calculation of the difference in pressures at the front axle between the wheels on the high and the low-coefficient-of-friction side results in that the detection occurs late. Therefore, the steering intervention cannot be initiated sufficiently early. The braking effect is hence not optimal. The risk of erroneous detection is also given because the wheel behavior is not analyzed. An erroneous YTL detection impairs the comfort or even the stability of the vehicle in an autonomous steering intervention.
In view of the above, it is an object of the invention to provide a μ-split detection in the anti-lock controller which can be used by an active steering system.